Propene metathesis reaction on di- and trinuclear molybdenum complexes grafted on mesoporous FSM-16 and silica - Structural characterization and their catalytic performances

Some polynuclear molybdenum complexes such as Mo-2(OAC)(4), Mo-2(OBut)(4), [Mo3O(CCH3)(OAc)(6)(CH3OH)(3)]Cl, and [(RhCp*)(4)Mo4O16] grafted with silica and mesoporous channels of FSM-16 were investigated for their structural characterization and catalytic performances in propane metathesis reaction. It was demonstrated by ESR, FT-IR, and EXAFS studies that Mo-2(OAc)(4) grafted on FSM-16 while keeping a dinuclear Mo-Mo unit by a partial removal of the acetate ligand at 623 K, resulting in the formation of mixed valent dinuclear species [Mo-Mo](V) characteristic of an intense ESR signal (g = 1.926) and EXAFS parameters (R (Mo-Mo) = 2.11 Angstrom coordination number = 0.6), similar to those grafted on silica, The resulting Mo dimer species-grafted on FSM-16 exhibited a high activity in propene metathesis to yield ethene and 2-butenes. On the other hand, FT-IR, TPD/MS and EXAFS studies revealed that [Mo3O(CCH3)(OAc)(6)(CH3OH)]Cl on FSM-16was transformed by pyrolysis at 573 K to Mo trinuclear species which is active for propene metathesis. The relative activities of surface-grafted Mo species grafted on FSM-16 were measured as the following order with the;respect to molybdenum nuclearity of the precursor complexes; Mo-2 (4100) > Mo-3 (110) > Mo-4 (7.3) much greater than Mo-7 O-24(2-) (0.3), where the numbers in parentheses denote TOF (h(-1)). The exceeding thermal treatment above 673-723 K for the active Mo-2 and Mo-3 species causes the complete cleavage of the Mo-Mo bond to give highly dispersed MoO3 on silica and FSM-16, which is inactive for propene metathesis. By contrast to silica and FSM-16, the Mo-2 and Mo-3 catalysts prepared on Al2O3, MgO and NaY are inactive for propene metathesis. (C) 2000 Elsevier Science B.V. All rights reserved.